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While Charged Lepton Flavor Violation (CLFV)
is not explicitly forbidden
in the Standard Model (SM) of
particle physics,
it is dynamically suppressed and CLFV processes
remain unobserved. Although the SM is very well tested in many regimes, it appears likely to be
incomplete. In many of the Beyond the Standard Model (BSM) scenarios,
rates for CLFV processes are within
the reach of the next generation of experiments. In particular,
if SUSY particles
have masses and couplings within the discovery reach of the Tevatron
or the LHC, many scenarios predict easily observable CLFV rates.
Moreover, many CLFV searches have a sensitivity to new physics that exceeds
the reach of even the LHC.

One example of a CLFV process is neutrino-less muon-to-electron
conversion in the Coulomb field of
a nucleus. To search for this process, a beam of slow muons is stopped
in a series of thin target foils and individual muons are captured into
atomic orbits. When muons decay in orbit, the electron daughters
have a continuous energy spectrum with an endpoint just less than the muon
mass (not half of the muon mass - radiative corrections are important).
In the conversion process, however, the final state has just two bodies: an
electron and a recoiling, intact, atomic nucleus.
The electron from this process is mono-energetic with an energy equal to
endpoint-energy of the continuous spectrum. The heart of the Mu2e
detector is a low-mass
magnetic spectrometer which can measure the electron momentum with a resolution
of order 0.15%. When the Mu2e experiment is completed, the collaboration
will publish the
measured energy spectrum of electrons: will there be an excess
at the endpoint? Stay tuned!

The description of the experiment in the previous paragraph began with a
beam of
slow muons. One of the technical challenges in the Mu2e experiment is to
build the muon beam line that will deliver a beam of slow
muons onto the target foils.
The Fermilab accelerator complex will deliver an 8 GeV (kinetic energy)
proton beam onto a
target housed inside a system of curved solenoids with a graded magnetic
field.
Interactions between the protons and the target nuclei produce pions that are
captured by the solenoids. When those pions decay to muons, the muons
are also captured by the magnetic field and are transported to the target
foils.

One of
the advantages of siting Mu2e at Fermilab is that the experiment will reuse,
with minimal modifications, many parts of the accelerator complex that will
become available following the completion of the Tevatron program.
Never-the-less some signifcant R&D challenges remain, one example
being how to ensure the stability of resonant extraction with the high beam
intensities required for Mu2e.

Additional details about the Mu2e experiment, from the proton beam to
projections of the physics sensitivity, can be found by following the
links to the
left; the most complete information can be found in the proposal. Some of
the other information is still under development.